Initial enthusiasm with optical fiber, when first introduced, was soon dampened by a number of considerations. Importantly, early fiber structures—of core size sufficient to support many modes of light—were unexpectedly capacity-limited. It was found that pulse repetition rate in such fiber was limited by pulse spreading, which could obliterate the unoccupied pulse positions that represented the information content of the pulse stream. Laser-generated pulses, single-mode as introduced, were found to have significant higher-mode content as received, with differing modal group velocities accounting for the observed pulse spreading. Intense effort was unsuccessful in avoiding fiber imperfections—e.g., the scattering centers—which had been identified as a primary cause responsible for mode conversion and, accordingly, for generation of the higher-order modes.
A promising remedy was suggested by S. E. Miller. As described in his U.S. Pat. No. 3,966,446, Jun. 29, 1976, a multitude of “perturbations” in composition were to be made along the length of the fiber. The objective was an introduction of localized abrupt changes in index-of-refraction, to serve as additional mode conversion centers. The intent was to increase incidence of mode conversion to an extent that all modal groups, upon arrival, would have undergone many mode-to-mode transformations during transit, thereby “averaging” group velocities, narrowing pulse width, and retrieving lost information capacity. Miller's work, in which he perturbed the preform glass along its length by changing dopant content, thereby producing corresponding index variations in the drawn fiber, confirmed expectation, and others continued the effort.
H. M. Presby, U.S. Pat. No. 4,038,062, Jul. 26, 1977, is illustrative of further work directed to mode-mixing. That patent teaches use of a pulsating heat source that plays directly on the fiber as it is drawn from the preform, thereby periodically varying axial alignment and/or diameter, and, in this manner, introducing controlled periodic index fluctuations, as “seen” by the signal.
However, while mode coupling had the desired effect of increasing bandwidth, it was invariably accompanied by increased power loss, and commercialization was limited. Today's state-of-the-art multimode fiber systems continue to use unperturbed fiber.
Recognizing the added fiber loss to be due to increased coupling between (supported) core modes and (unsupported) cladding modes—an unwanted effect accompanying the (wanted) increased coupling between core modes—fiber structures, restricting induced coupling to core modes, were considered. Two approaches, together, illustrate the intensity of the effort and the sophistication of the reasoning entailed in the effort to design a fiber that would enjoy the advantage of mode-mixing without the cost penalty of increased loss.
U.S. Pat. No. 3,909,110, issued to Dietrich Marcuse on Sep. 30, 1975, makes use of an inherent property of multimode fiber for differentiating between core modes and cladding modes in step-index fiber. Recognizing that positions of high field intensity, associated with successively higher-order modes, lie at successively increased radial spacing from the fiber axis, the inventor would now be enabled to selectively couple (lower-order) modes. This he sought to do by localizing perturbation-index changes on or near fiber axis where the fields of the lower-order bound modes are concentrated. Unfortunately, while lessening coupling efficiency for higher-order modes in this manner, mode mixing with still non-zero fields on or near-axis continued, with associated unacceptable added loss.
U.S. Pat. No. 4,176,911, issued to Marcatili et al. on Dec. 4, 1979, describes an effort to avoid the added loss associated with mode mixing in the parabolic-index core, multimode fiber structure, which was gaining favor over the step-index structure with which Marcuse was concerned. The parabolic (or “alpha”) profile tends to equalize transit times for the various modes, thereby contributing to pulse-narrowing, and continues to be favored to this day. Unfortunately, to the extent that profiling accomplishes this purpose, it tends also to equalize radial spacings between successively higher order mode pairs. This, in turn, tends to equalize coupling probability for all core modes and eliminates the inherent differentiation offered by step-index fiber. The inventive solution was a departure from the usual “matched clad”, in which the index-v-radius parabolic profiling of the core was continued, without interruption, into the cladding. Instead, the invention provided for a core-cladding “mismatch”—for an abrupt index decrease at the core-cladding interface, resulting in a cladding of index value markedly less than attained in the core—for the purpose of separating core and cladding modes. The well-reasoned approach reduced—but did not sufficiently eliminate—added loss.
Interest in multimode fiber design waned with commercial introduction of single-mode fiber—with its fiber core supporting only the fundamental mode, and, so, avoiding mode dispersion altogether. Advancing technology had enabled manufacture of fiber structures with the needed degree of control for making cores of the necessary 1-6 μm radius. (State-of-the-art, silica-based multimode fiber is characteristically of core radius of at least 25 μm—common designs have 50 μm or 62.5 μm cores.) Single-mode fiber continues to dominate the all-important long distance communication market.
The fiber art has made impressive advances. Low-loss dopants/doping processes, for tailoring index-of-refraction and imparting wanted light-guiding properties, have been developed. Intractability of the high-melting, and easily-contaminated, silica-based fiber has yielded to a number of suitable manufacturing processes, which maintain product within extremely tight compositional, dimensional, and purity specifications.
Common manufacturing processes are: Modified Chemical Vapor Deposition, and the “soot” processes—Outside Vapor Deposition and Vapor Axial Deposition. As described in Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, 1979, Academic Press, in Chapter 8, all of these processes react gaseous silicon halide-containing material with oxygen, to produce initial, silica-containing, particulate material of carefully-controlled composition, which, as consolidated, yields at least the critical core portion of the preform from which the fiber is ultimately drawn. MCVD and OVD achieve critical core profiling by means of layer-by-layer, longitudinal deposition of thin layers of material of differing-composition—of material containing varying kind and/or amount of index-increasing or index-decreasing, dopant. VAD depends on “end-on” deposition of material of radially graded composition for profiling. Preform preparation may entail further processing such as etch-removal of temporary substrate—of the enclosing MCVD deposition tube or the enclosed OVD mandrel. Resulting hollow MCVD and OVD bodies are subsequently collapsed to yield the preform from which the fiber is drawn. MCVD manufacture lends itself to a cost-reducing procedure, by which the consolidated body is placed within an outer cladding tube of less critical, relatively inexpensive material, to produce the (now composite) preform.
Other process characteristics may require attention both by designer and manufacturer. The high-temperature, preform collapse of both MCVD and VAD, may result in some “burnout” of index-increasing dopant and in a consequent “central dip” of reduced index-of-refraction along the fiber axis. Multiple torch passes for layered deposition generally result in some “outgassing” of more-volatile dopant at layer interfaces, and in consequent “profile ripple”.
Single-mode fiber and systems, in retaining dominance, have undergone many iterations. Dispersion-Shifted Fiber eliminated chromatic dispersion at the operating wavelength, thereby avoiding what was regarded as the remaining cause of pulse-broadening in single-mode systems. Such DSF was, in turn, superseded by Non-Zero Dispersion Fiber, providing for reduced but well-controlled finite values of chromatic dispersion, to permit high bit-rate, individual-channel operation while, at the same time, enabling high-capacity, multi-channel (“Wavelength Division Multiplex”) systems. Such NZDF provides an appropriate balance between: (a) low values of chromatic dispersion commensurate with high, per-channel bit-rate, and (b) needed chromatic dispersion for lessening the effect of 4-photon mixing (4PM)—a nonlinear dispersion mechanism introduced in WDM operation, and causing a type of channel-to-channel “cross talk”. (Total absence of chromatic dispersion in DSF eliminated periodic phase cancellation, thereby permitting unlimited buildup of spurious signal and precluding the increased capacities expected from multi-channel operation. Operation with a trillion bit/sec capacity on a single NZDF fiber has been demonstrated.)
Multimode fiber has, however, maintained a niche position for use for shorter-distance communication. This is due, in part, to commercial unavailability of single mode fiber suitable for operation in the 800-900 nm wavelength range traditionally used in such systems. Replacement of such multimode fiber by the 1310 nm or 1550 nm single-mode fiber used in long distance systems, requires replacement of terminal equipment, and is costly. Other cost considerations favoring multimode fiber systems are: lower packaging costs for optoelectronic sources and detectors; and lower interconnection costs for fiber splices and connectors.
Multimode fiber has also retained a significant presence in “private networks”—in local area networks (LANs)—where the very high bandwidth offered by single-mode fiber has not been an issue.
Technological advance has benefited multimode fiber to some extent. Incidence of scattering centers has been lowered to the extent that associated loss in regularly produced fiber—multimode as well as single-mode—has been reduced by a factor of 4, to a value below 0.5 dB/km. In multimode fiber serving as mode-conversion centers, this is accompanied by some lessening of pulse-spreading. Further, the alpha-profiled core of state-of-the-art multimode fiber reduces velocity differences for supported modes, also lessening pulse-spreading.
Industry concentration on single-mode fiber and systems has, however, resulted in significantly increased fiber capacity—in large part, evidenced by improvements in Dense Wavelength Division Multiplexing. At the same time, desire for increased LAN system capacity has grown. As a result, that niche position, for so long held by multimode fiber on basis of cost considerations, is threatened. Many expect next-generation LANs to be single-mode.